One of the most complex forms of congenital heart disease, hypoplastic left heart syndrome (HLHS), affects an average of 1 in every 5,000 newborns with a mortality rate of up to 35 percent during the first year of life. The overall goal f our research is to develop safe and effective ways to improve short-term and long-term survival rates in babies born with HLHS. We aim to introduce a novel cell-based therapeutic strategy in the three-stage surgical procedure that HLHS patients typically undergo. Our hypothesis is that injected allogeneic cardiosphere derived cells (CDCs) will boost and regenerate the myocardium of the single right ventricle in HLHS patients, improving right ventricular function and, thus, short and long-term clinical outcomes. The aims of this Phase I/2 clinical trials are to evaluate the feasibility and safety of direct intra-coronary injection of allogeneic CDCs in these HLHS patients at the time of cauterization for their second operation. This proposal will translate a series of basic and pre-clinical observations into a potentially paradigm-shifting therapy to meet an unmet medical need in HLHS patients. In addition, this study will bring a deeper insight into the biology of stem cells in human patients which have never been studied before in pediatric congenital heart patients.